The bolted joint is a very important fastening method in engineering assemblies. It works by screwing together two or more parts with a bolt and nut. The bolt or nut may be made integral with one of those parts or the bolt may pass through a bore in both parts to engage the nut on the opposite side. The screwing action acts through co-operating threads to draw the bolt head and nut together so that their faces produce a clamping force on the parts between them. As the nut is tightened the material of the bolt shaft stretches and the material forming the parts being fastened compresses so that the tension force in the bolt equals the compression force in the joint. The clamping force should be kept within defined limits: if the clamping force is too low, the joint will be loose; if the clamping force is too high, there is a risk that the fastening may fail or the joined components may be damaged.
Although it would be desirable to measure the clamping force directly, it is very difficult to measure the precise extent of the stretching or compression in the joint, and therefore to deduce the magnitude of the clamping force. Experiments are therefore performed with force washers to arrive at a corresponding value for the torque applied to the fastener, which is easy to measure as a surrogate for the actual clamping force. Once that torque value has been established for a given type of fastener, it may be replicated as a reliable means of creating a bolted joint with a known clamping force, between specified limits. However, to replicate reliably the amount of torque imparted during tightening of the joint, it becomes essential that the rotary fasteners used to tighten the joints are also periodically checked, to make sure they give a correct indication of torque before they are used on an assembly line.
International standards have been set up to specify performance test routines for checking the calibration of rotary tools before they are used on assembly lines to fasten bolts and similar threaded fasteners. These performance test methods use Joint Rate Simulators (JRSs), which simulate the torque pattern that is experienced as a joint is tightened. To a first approximation, as a typical joint is tightened, the torque increases linearly with the angle turned by the screw thread. A JRS uses this characteristic to provide a test piece on which the tool will fit, such that when the tool applies torque to turn the test piece, that torque increases with the angle through which the test piece turns. The rate of increase of torque with increasing angle is referred to as the torque rate. The angle through which the joint must be turned to tighten it fully depends on the torque rate of the joint and on the torque applied. For a “hard joint”, full tightening is accomplished in a fraction of a revolution. In contrast, in a “soft joint”, full tightening is accomplished over a much greater angular range of movement, possibly several complete several revolutions of the fastener.
A tool is tested by setting the JRS to the desired level of torque rate and applying the tool to the bolt head of the JRS via an intermediate torque sensor. For the desired level of torque rate, the torque reading of the tool can be compared with the torque reading of the torque sensor to confirm that the tool is measuring torque correctly or to calibrate the tool. Any complete performance testing routine comprises a number of repeated tightening cycles of the test joint, with the results being averaged or statistically analyzed.
International patent application WO 2008/099204 describes a variable torque rate test joint (i.e. a JRS), which comprises a bolt that screws into a threaded bore of a test rig. A torque rate adjustment device comprises at least one spring beam that is anchored to a reaction point at one end and flexes as it extends in cantilever over a pivot point. The other end of the beam exerts axial pressure on a collar that acts against the head of the bolt to provide a force that resists turning of the bolt. The amount of flexure can be adjusted to vary the axial force and hence the torque rate of the test joint by varying the distance between the pivot point and the rotary axis of the bolt. That test joint was an improvement over earlier test joints because the test bolt upon which the tool acted had a relatively small moment of inertia, which was comparable to the actual moment of inertia of a genuine fastener bolt. Therefore the joint provided a more realistic simulation of the conditions under which the tool would be used. The correct moment of inertia is a particularly important consideration when the test joint is used for the performance testing of impulse drive tools. These tools rely on the transfer of pulses of torque, each pulse being a few milliseconds in duration, with many pulses per second being applied to the joint. If the joint has a large moment of inertia, then the tool cannot transfer enough energy to make the joint initially free-turn before the joint tightens, and the tool can then stall. A JRS that has a moment of inertia that is not matched to that of the real joint under simulation may thus not give true results for impulse tools.
In the test joint described in WO 2008/099204, the test bolt must have a certain minimum length in order to pass through the thickness of the apparatus. Therefore the moment of inertia of the test bolt cannot be reduced indefinitely and that type of test joint cannot be used to simulate smaller nuts and bolts accurately. A further problem is that the torque sensor located between the rotary tool and the test bolt adds to the moment of inertia of the moving parts upon which the tool must act.
As previously indicated, a testing programme for a rotary tool typically involves a large number of individual tests over which the results are averaged, for example a series of 50 tests. WO 2008/099204 describes that, at the end of each test, the clamping force on the test joint may be released and means such as a return spring or an electric motor may be used to rotate the test bolt back to its starting position for a further test in the series to be carried out. However, many rotary tools cannot be rotated in reverse so they would have to be removed from the bolt head after each test, which is laborious and inefficient. A better way of resetting the test joint at the end of each test in the series is therefore needed.
The measurement of torque is not a perfect substitute for the measurement of clamping force, which is the real quantity of interest. The relationship between torque and clamping force may vary with temperature and as a result of varying friction between the joint components. Friction depends on the amount of lubrication and may also change as the joint ages and becomes worn smooth by repeated use, which is a particular problem in a test joint. It is therefore desirable to provide more direct measurements of the clamping force and/or to be able to compensate for deviations from the expected torque rate of the test joint.